The organisation of the basal ganglia in the domestic chick (Gallus domesticus): anatomical localisation of DARPP-32 in relation to glutamate.

Subpallial structures are highly conserved across the different vertebrate species. They are instrumental in the neural processing relevant to adaptive learning, decision making, motivation and behavioural strategies. Of the striatal regions, our attention has been focussed on the medial and ventral striatum (MSt), now parcellated into subregions, and also including the nucleus accumbens (Ac). Similar to mammals, the avian Ac and MSt receive glutamatergic input from the pallium and dopaminergic input from the substantia nigra and ventral tegmental area. Coincidence between glutamatergic and dopaminergic synaptic activities in the ventral/medial striatum, including the Ac, is required for memory to be formed for a given pairing of stimulus and a hedonic quality or behavioural salience. The underlying mechanism involves the activation of NMDA and dopaminergic receptors, as well as the phosphorylation of dopamine-cAMP-regulated phosphoprotein (DARPP-32). Using quantitative electron microscopy of chick specimens double-labelled against glutamate and DARPP-32 we observed direct synaptic connections between glutamate immunoreactive axon terminals and DARPP-32 labelled dendrites in the MSt and also in the posterolateral telencephalon (nidopallium caudolaterale, a prefrontal cortex equivalent region) and the hippocampus. Glutamate immunoreactive axons synapsed with both DARPP-32 immunoreactive (DARPP-32+) and DARPP-32 negative (DARPP-32-) dendrites, forming asymmetrical junctions, in all brain regions observed. The existence of direct synaptic contacts between excitatory amino acid containing axon terminals and DARPP-32 containing dopaminoceptive neurons of the chicken MSt underlines the functional homology with mammalian striatal systems.